i owned a 4870 and currently a gtx260-216. both were factory overclocked. there are games that favor ati, there are games that favor nvidia.
but the 260-216 (gt200b 55nm) is the better buy (assuming the price difference is not beyond $10) as you can OC it to reach 4890/gtx275 speeds. that will be somewhere around 680mhz and above on the core. im not brave enough to push it beyond 700mhz though as i only have the reference cooler. the 4870 on the other hand, you'll be lucky to get past 820 mhz without crashing/artifacting.